In the majority of oil wells, the fluid is produced by a lift pump which delivers the fluid from the formation to the surface through a tubing string. Generally, the pump is of the reciprocating type which is driven by a rod extending through the tubing. However, electric pumps may also be used. As fluid is removed from the wellbore, fluid flows from the surrounding formation into the wellbore due to the higher pressure in the formation. The wellbore pressure at the formation is the summation of the casing pressure, the gas column pressure and the liquid column pressure above the formation. If the pump does not have sufficient capacity to remove all of the liquid from the wellbore, a column of liquid or a high casing pressure will result. This causes an increase in the wellbore pressure and thus restricts the flow of fluid from the formation into the wellbore. Under these conditions, the maximum fluid flow into the wellbore does not occur. In most cases, an operator desires to obtain the maximum production from a well and hence he desires to maintain the liquid level near the formation with a minimum of casing pressure. The bottomhole pressure should be maintained at a minimum value compared to the reservoir pressure in order to obtain the maximum production from the well.
A measurement of the depth to the liquid level can be made to aid in the determination of the producing rate efficiency of a well. In addition, consecutive, periodic fluid level tests and casing pressure measurements can be performed to obtain the pressure buildup in the wellbore when a well is shut-in. This information allows the operator to determine several important reservoir and wellbore characteristics. Thus, the operator has need for a knowledge of the distance to the liquid level, and preferably this information should be presented automatically in a display or printout convenient for the operator.
The use of echo sounding to determine fluid depth is well-known in the art. This is shown in U.S. Pat. Nos. 2,190,141 to Walker, 2,232,476 to Ritzman, and 4,318,298 to Godbey et al. It is well known that an acoustic pulse can be transmitted down the borehole and reflections from the collars can be counted. By determining the number of collars between the wellhead and the surface of the fluid, a calculation can then be made of the depth to the fluid surface knowing the average length of each tubing joint. One example of a current echo sounding system in wide use is a Model D Echometer made by Echometer Company of Wichita Falls, Tex. This device produces a strip chart record of the reflection data, but the operator is tasked with the job of selecting the reflection from the surface of the liquid and counting the collar reflections from the tubing collar by use of a mechanical spreader.
Acoustic pulse generators, typically called "guns", for use in echo sounding are shown in U.S. Pat. Nos. 4,637,463, 4,408,676, 3,915,256, and 3,316,997 all to McCoy.
Prior art patent U.S. Pat. No. 4,793,178 to Cebuhar et al. describes the process of echo ranging and includes a technique for automatically detecting the liquid level echo in a reflection signal.
In conventional echo sounding techniques, the signals from the collar reflections often disappear in the noise before the liquid level reflection is received. Conventional filtering can reduce the noise level and extend the extraction of the collar reflections to some extent, but in many applications the reflections, even in a filtered signal, cannot be completely resolved.
The difficulty of determining the existence of reflections from collars is most acute in deep wells or wells in which there is considerable noise. In addition, there can be numerous other types of reflectors downhole, such as tubing anchors, perforations and deposits. These reflections can produce a signal that is similar to a liquid level reflection. In view of these difficulties, there exists a need for a method of processing, filtering and displaying echo sounding data in such a way to better detect and count the collar reflections, detect the liquid surface reflection and allow the operator to select the liquid surface reflection from among a number of similar reflections.